Nanocrystalline silicon (nc-Si) particles have attractive characteristics for a luminescence phenomenon. The luminescence color of nc-Si particles is dependent strongly on the particle size because the reduction of particle size lead to the increase of luminescence energy. The particle size can be freely adjusted by the oxidation onto the surfaces of nc-Si particles. Here, we report the photoluminescence (PL) property from the surface oxidized nc-Si particles in pure water.

The nc-Si particles were formed in a silicon dioxide (SiO2) layer by cosputtering of Si chips/SiO2 targets and subsequently annealing at 970 oC. The SiO2 layer surrounding the nc-Si particles, then, was etched by the hydrofluoric steam treatment. After etching, the nc-Si particles were uniformly dispersed in pure water by the ultrasonic wave treatment. The samples were evaluated by using a high resolution transmission electron microscope, fourier transform infrared and PL measurements.

The nc-Si particles with an average size of approximately 2.5 nm were existed in pure water. The surfaces of nc-Si particles were adsorbed with hydrogen and oxygen atoms. The nc-Si particles emitted a red light with a peak at 720 nm. When the nc-Si particles were immersed in pure water during 250 hours, the average size of nc-Si particles reduced from 2.5 nm to 2.3 nm by the increase of oxidation quantity on the surface. This was caused by the replacement of Si atoms existing mostly outside region of nc-Si particles and oxygen atoms which dissolve in pure water. The reduction of particle size resulted in the variation of luminescence color from red light (720 nm) to orange light (650 nm).